The reason why a Ball Grid Array (BGA) package has become a mainstream of the package products these days is mainly because of its sufficient provision of I/O connections to meet the demands of semiconductor chips on which a high density of devices and circuitry is integrated. However, the more devices and circuitry that are integrated on a semiconductor chip, the more heat that is generated. Without dissipating the heat generated by the semiconductor chip in a timely manner, the life and performance of the semiconductor chip will be greatly reduced.
In order to solve the above-mentioned drawbacks, the idea of adding a heat sink into a semiconductor package has thus arisen. The technique is to wrap the semiconductor chip together with the heat sink inside molding compound, after the semiconductor chip has been adhesively positioned on the heat sink. Although the method for wrapping the heat sink into the molding compound helps to enhance the heat dissipation rate, it increases the overall height of the semiconductor package. At the same time, the dissipation path for the heat generated on the surface of the semiconductor chip requires the heat to move from the semiconductor chip to the heat sink and then through the molding compound, where it finally dissipates into the ambient environment. This thermally conductive path is too long and, moreover, the heat needs to pass through molding compound having a low heat dissipation rate, It is thus very hard to enhance the overall heat dissipation rate of this idea.
To address these drawbacks, U.S. Pat. No. 5,642,261 discloses a semiconductor package, in which the substrate has a cavity to accommodate a heat sink. As shown in FIG. 4, the heat generated on the semiconductor chip is directly transferred to the printed circuit board (PCB) connected with the semiconductor package through the heat sink having a large heat dissipation area. Although such a structure may enhance the performance of heat dissipation rate without increasing the overall height of the semiconductor package, such a semiconductor package requires forming an opening through the substrate so as to insert a heat sink therein. By connecting the chip with the heat sink, the heat generated by the chip is then directly dissipated to the atmosphere. Nevertheless, since an opening with a fixed size is required to be formed on the substrate to insert a heat sink, the size of the opening must coincide with the size of the heat sink to prevent the humidified air outside from entering inside of the package through the gap formed between the substrate and the heat sink. This increases the requirements of manufacturing precision and difficulties during operation. Moreover, since the coefficient of thermal expansion of the substrate and the coefficient of thermal expansion of the heat sink are often significantly different from each other, the effect of thermal stress during a thermal cycle and a reliability test may induce cracking at the connecting surface of the substrate and the heat sink. Humidified air may thus enter inside the package through the gap formed between the substrate and the heat sink and affect the reliability of the semiconductor package.